GMG said the decision was made as MediaCom has an in-depth understanding of the key markets in which GMG radio stations broadcast.
GMG Radio's stations broadcast under the Smooth, Real, Century and Rock Radio brands. The group is launching Scottish radio brand Rock in Manchester early next summer.
The agency pitch process was overseen by GMG Radio commercial director Stuart Taylor and group marketing manager Helen Dickinson.
Taylor said: "Smooth Radio is breaking new ground in targeting the 40 to 59-year-old listener. Reaching this market will be a key focus for activity in the coming year."
GMG Radio is also staging a pitch for the creative account for its Smooth Radio brand, which represents six of its stations. It will hold a pitch for this business in January, with a decision expected by the end of the month.